The running time of an ultrasound pulse, for the distance to a preceding vehicle and back, can be measured for the determination of the distance of one vehicle from another vehicle. To accomplish this, the ultrasound transducer generates an ultrasound signal in pulse form. An ultrasound sensor records the reflected components of the ultrasound signal, and a post-connected device determines the running time. From the known speed of sound of approximately 330 m/s and at a distance from the front vehicle of approximately 7.5 cm, there follows an echo time of 0.5 ms.
The ultrasound signal is typically generated by an ultrasound transducer device, in which a piezo actuator actuates a pot diaphragm. A voltage signal having a frequency in the ultrasound range (>20 kHz) and an amplitude in the medium voltage range (10 V-200 V) is applied to a piezo actuator. An electronic wiring configuration of such an ultrasound transformer device 5 is shown, with reference to FIG. 6. A current source 2 generates a transmitting current 1 (signal), which is transmitted inductively via a primary inductance 4a of a transformer device 4 to a secondary inductance 4b. In this context, transmitting current 1 is shifted by switch 3 which, after a transmitting excitation, transfers into a high-resistance center position. Secondary inductance 4b, together with the ultrasound transducer device and its electrical capacitance, forms an anti-resonant circuit having an electrical resonant frequency. The electrical resonant frequency is in the ultrasound range. In order to achieve an optimal excitation of the piezo actuator, the electrical resonant frequency is tuned to the mechanical resonant frequency of sound transducer device 5.
Sound transducer device 5 is used not only for transmitting acoustical ultrasound signals 11 but also for receiving reflected component 12 of ultrasound signal 11. Received reflected ultrasound signal 12 is converted to a voltage level by the sound transducer device 5, and is amplified via an amplifier device having an operational amplifier 6 and output as output signal 10.
The component of reflected ultrasound signal 12 of transmitted ultrasound signal 11 is very small. Accordingly, only very low voltage levels come about during the conversion of the reflected ultrasound signal by the sound transducer device. These are typically in a range of a few 10 μV at the amplifier input. In order to be able to record these signals, it is necessary for the voltage amplitude of the oscillation in the anti-resonant circuit to have fallen to a lower level than the voltage potential of the received signals, during transmission of ultrasound pulse 11. To achieve this, a sufficiently strong attenuation of the anti-resonant circuit is required. Attenuation takes place via a resistor R1, which is connected to the inverting input of the operational amplifier. As long as operational amplifier 6 is not driven to saturation by the signal amplitude of the anti-resonant circuit, attenuation of the anti-resonant circuit comes about which is proportional to resistor R1 and the voltage amplitude of the oscillation in the anti-resonant circuit, because a current is flowing into the virtual ground at the inverting input of operational amplifier 6. In the case of a typical amplification of the feedback operational amplifier circuit of 10, supply voltages of the operational amplifier of 0 and 5 Volt and a biasing voltage equal to one-half the supply voltage (2.5 V) at the non-inverting input of the operational amplifier, a saturation of the operational amplifier takes place at the latest in response to a signal having an amplitude of 250 mV. The saturation starts correspondingly earlier if the maximum modulation level of the operational amplifier is lower than the supply voltage.
An inverting input of operational amplifier 6 is typically connected via damping diodes D3 and D4 to a ground potential Gnd and VDD in the blocking direction. In addition, the anti-resonant circuit is connected to the operational amplifier via a capacitor C1, in order to achieve isolation of the DC voltage levels. For signal amplitudes of the anti-resonant circuit whose amount is greater than the sum of one-half the supply voltage VDD/2 (2.5 V) and the voltage drop over diodes D3 and D4 (0.6 V), there comes about a current flow to supply potential VDD and ground potential Gnd via resistor R1. For large amplitudes, in the previous example, for amplitudes amounting to 3.1 V, an attenuation comes about in this way for the anti-resonant circuit which is proportional to the amplitude of the signal in the anti-resonant circuit and proportional to resistor R1. For signal amplitudes in the range between 0.25 V and 3.1 V, however, there is no attenuation that is a linear function of the signal amplitude in the anti-resonant circuit, because, on the one hand, the clamping diodes do not conduct yet, and on the other hand, the operational amplifier is in saturation.
Instead of a clamping of the inverting input of the operational amplifier using diodes, a pair of transistors of two types of conductivity can be used in a push-pull configuration. In this context, the emitter-collector path of the one transistor connects ground to the inverting input and the emitter-collector path of the other transistor connects the supply voltage to the inverting input. Half the supply voltage is present at the base of the transistors. Because of that, one of the two transistors switches to conductive when the amount of the signal amplitude becomes greater than 0.6 V. There comes about, in this instant, a non-linear, attenuated range of 0.25 V to 0.6 V.